borg2: borg transfer fails with zlib compressed archive in borg 1.x repo

[ticpu]

Have you checked borgbackup docs, FAQ, and open GitHub issues?

Yes

Is this a BUG / ISSUE report or a QUESTION?

BUG

System information. For client/server mode post info for both machines.

Your borg version (borg -V).

❯ borg -V
borg 2.0.0b7

Operating system (distribution) and version.

Arch Linux

Hardware / network configuration, and filesystems used.

It is an external USB disk with a btrfs GPT partition with default options. Btrfs scrub reports no error.

How much data is handled by borg?

Around 1.5TB.

Full borg commandline that lead to the problem (leave away excludes and passwords)

❯ borg --repo /mnt/custom/borg2 transfer --other-repo /mnt/backups/borg --compress=zstd,10 --recompress=always --upgrader=From12To20

Describe the problem you're observing.

borg transfer from old repo to new is failing with an AssertError. The new repo was created using
borg --repo /mnt/custom/borg2 rcreate --other-repo=/mnt/backups/borg -e repokey-aes-ocb

Can you reproduce the problem? If so, describe how. If not, describe troubleshooting steps you took before opening the issue.

Yes, everytime I run the transfer command without --dry-run
Since the exception contains messages about cache, I tried rm -rf ~/.cache/borg and it doesn't help.

Include any warning/errors/backtraces from the system logs

downloads-2018-01-05 01:43:44.712277: archive is already present in destination repo, skipping.
home-2018-01-05 11:20:02.644243: copying archive to destination repo...
Local Exception

Error:

AssertionError: 

If reporting bugs, please include the following:

Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/borg/archiver/__init__.py", line 630, in main
    exit_code = archiver.run(args)
                ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/borg/archiver/__init__.py", line 536, in run
    return set_ec(func(args))
                  ^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/borg/archiver/_common.py", line 223, in wrapper
    return method(self, args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/borg/archiver/_common.py", line 156, in wrapper
    return method(self, args, repository=repository, cache=cache_, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/borg/archiver/transfer_cmd.py", line 131, in do_transfer
    chunk_entry = cache.add_chunk(
                  ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/borg/cache.py", line 954, in add_chunk
    cdata = self.repo_objs.format(id, meta, data, compress=compress, size=size, ctype=ctype, clevel=clevel)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/borg/repoobj.py", line 40, in format
    assert isinstance(meta, dict)
AssertionError

Platform: Linux p4 6.5.1.arch1.r1204752.b56e84bccab0 #1 SMP PREEMPT_DYNAMIC Mon, 25 Sep 2023 01:09:30 +0000 x86_64
Linux: Unknown Linux  
Borg: 2.0.0b7  Python: CPython 3.11.5 msgpack: 1.0.5 fuse: llfuse 1.5.0 [pyfuse3,llfuse]
PID: 3900158  CWD: /mnt/backups
sys.argv: ['/usr/bin/borg', '--repo', '/mnt/custom/borg2', 'transfer', '--other-repo', '/mnt/backups/borg', '--compress=zstd,10', '--recompress=always', '--upgrader=From12To20']
SSH_ORIGINAL_COMMAND: None

[ThomasWaldmann]

What kind(s) of compression did you use in the old repo (for archive home-2018-01-05 11:20:02.644243)?

Did you use the obfuscation feature?

Update: guess it was legacy zlib compression (which was kind of a special case and there is a bug dealing with that). see fix in #7884.